Dimming apparatus for fluorescent lamps

ABSTRACT

The present invention is to provide a fluorescent lamp dimming apparatus capable of continuously dimming the fluorescent lamp, and capable of eliminating a fluctuation of light outputs, and in particular, capable of turning ON the fluorescent lamp in a low temperature. The fluorescent lamp dimming apparatus is comprised of a lamp voltage detecting circuit for detecting a discharge voltage of the fluorescent lamp, and a control unit intermittently controls the output frequency of a high frequency power supply in such a manner that the output frequency becomes higher than a dimming frequency based on the dimming signal so as to periodically change a current supplied to the fluorescent lamp into a low current; and when the output frequency of the high frequency power supply becomes higher than the dimming frequency, the control unit sets a lower limit dimming set value based upon a detection voltage of the lamp voltage detecting circuit in order that the output frequency of the high frequency power supply is controlled to become lower than, or equal to the upper limit frequency in accordance with the lower limit dimming set value.

TECHNICAL FIELD

The present invention relates to a dimming apparatus of a fluorescentlamp, and more specifically, to a technique for turning ON thefluorescent lamp under stable condition in a low temperature.

BACKGROUND ART

FIG. 23 is a block diagram for representing an arrangement of aconventional dimming (light controlling) apparatus of a fluorescentlamp, similar to the fluorescent lamp dimming apparatus described in,for example, Japanese Patent Application Laid-Open No. Hei 6-333692.

In FIG. 23, reference numeral 1 indicates a high frequency power supply,reference numeral 2 indicates a coil, reference numeral 3 shows afluorescent lamp (will be simply referred to as a lamp hereinafter),reference numeral 4 denotes a control unit of the high frequency powersupply 1, and reference numeral 5 represents a dimming (lightcontrolling) signal.

In the apparatus shown in FIG. 23, the control unit 4 controls thefrequency of the high frequency power supply 1 in response to thedimming signal 5 entered thereinto. Since the high frequency powersupply 1 is connected via the coil 2 to the lamp 3, the impedance of thecoil 2 is varied by the frequency of the high frequency power supply 1.In connection with this impedance change, a high frequency currentflowing through the lamp 3 is varied. In other words, the higher thefrequency of the high frequency power supply 1 becomes, the smaller thecurrent flowing through the lamp 3 becomes, so that the lamp 3 isbrought into dimming states.

Since the dimming degree of the lamp 3 is substantially directly inproportion to the current flowing through the lamp 3, the control unit 4controls the frequency of the high frequency power supply 1 in responseto the inputted dimming signal 5 so as to dim the lamp 3. For the sakeof convenience, in the case that the dimming signal 5 indicates aspecific dimming degree, a frequency outputted from the high frequencypower supply 1 is referred to as a dimming frequency, and also, adimming degree at this time is referred to as a set dimming degree (%indication).

FIG. 24 is a graphic representation for showing an example of the outputfrequency of the high frequency power supply 1 with respect to the setdimming degree. The control unit 4 controls the high frequency powersupply 1 in such a way that when the set dimming degree is equal to, forexample, 100%, the frequency becomes 50 kHz, whereas when the setdimming degree is equal to 25%, the frequency becomes 80 kHz.

The light output from the lamp 3 under this control condition isindicated in a graphic representation of FIG. 25. In FIG. 25, 0° C., 10°C., and 25° C. represent atmospheric temperatures of the lamp 3. Evenunder the same set dimming degree, the light outputs are different fromeach other, depending upon the lamp atmospheres. This may be caused bythe characteristics of the lamp 3, namely the lamp impedance of the lamp3 owns the temperature characteristic. For instance, when the setdimming degree is equal to 100%, the light output becomes L1 at 25° C.;the light output becomes L2 at 10° C.; the light output becomes L3 at 0°C., and thus, a relationship of L1>L2>L3 is established.

Also, in such a case that the atmospheric temperature of the lamp 3 is25° C., the light output is continuously changed with respect to the setdimming degree. To the contrary, when the atmospheric temperatures ofthe lamp 3 are equal to 10° C. and 0° C., if the set dimming degree isdecreased, then the light outputs are rapidly changed, so thatdiscontinuous points appear.

As apparent from the above graphic representation, when the atmospherictemperature of the lamp 3 is 0° C., the light output is changed from apoint A to a point B in the vicinity of the set dimming degree of 40%(between 35% and 45%), and the light output becomes very small at thepoint B. Also, when the atmospheric temperature of the lamp 3 is 10° C.,a similar phenomenon occurs in the vicinity of the set dimming degree of30%.

This reason is given as follows. When the atmospheric temperature of thelamp 3 is low (less than or equal to 10° C.), the lamp voltage isincreased in the dimming degree lower than, or equal to a certaindimming degree, as compared with that of the normal temperature. Also,the lower the dimming degree becomes, the stronger the increasing trendof the lamp voltage is changed. When the lamp voltage is rapidlyincreased, the operating point of the current flowing from the highfrequency power supply 1 to the coil 2 and the lamp 3 becomes unstable.The lamp current is suddenly decreased by feeding such a loop that thecurrent flowing through the lamp 3 is decreased→the impedance of thelamp 3 is increase→the current flowing through the lamp 3 is decreased.

At this time, there are some possibilities that the light output isfluctuated, depending upon the condition of the lamp 3. Since the lampvoltage is low in the normal temperature (25° C.), the operating pointbecomes one, and the lamp current may flow under stable condition by thecoil 2.

Since the above-described conventional fluorescent lamp dimmingapparatus is arranged in the above-described manner, when the lampperipheral temperature is low, the light output is rapidly lowered,depending upon both the temperature characteristics of the lamp 3 andthe circuit condition from the high frequency power supply 1. As aresult, there are such problems that the lamp cannot be dimmed in thecontinuous manner, and also the light output is fluctuated.

The present invention has been made to solve these problems, and has anobject to provide a fluorescent lamp dimming apparatus capable ofcontinuously dimming the fluorescent lamp, and also capable ofeliminating fluctuations of light outputs.

DISCLOSURE OF THE INVENTION

A dimming apparatus of a fluorescent lamp, according to the presentinvention, is characterized by comprising: a fluorescent lamp; a highfrequency power supply for supplying high frequency power to thefluorescent lamp; a control unit for controlling an output frequency ofthe high frequency power supply in response to an inputted light dimmingsignal; and a coil provided between the high frequency power supply andthe fluorescent lamp, for limiting a current which flows from the highfrequency power supply to the fluorescent lamp; wherein: the dimmingapparatus is further comprised of a lamp voltage detecting circuit fordetecting a discharge voltage of the fluorescent lamp; and the controlunit intermittently controls the output frequency of the high frequencypower supply in such a manner that the output frequency becomes higherthan a dimming frequency based on the dimming signal so as toperiodically change a current supplied to the fluorescent lamp into alow current; and when the output frequency of the high frequency powersupply becomes higher than the dimming frequency, the control unit setsa lower limit dimming set value based upon a detection voltage of thelamp voltage detecting circuit in order that the output frequency of thehigh frequency power supply is controlled to become lower than, or equalto the upper limit frequency in accordance with the lower limit dimmingset value.

Also, the above-described control unit is characterized in that when aset dimming degree based upon the inputted dimming signal is smallerthan, or equal to a predetermined dimming degree, the control unitintermittently controls the output frequency of the high frequency powersupply in such a manner that the output frequency becomes a testfrequency higher than the dimming frequency from the dimming frequencybased upon the dimming signal.

Also, the above-described control unit is characterized in that when thedetection voltage of the lamp voltage detecting circuit is higher than apreset threshold value voltage, the control unit sets a lower limitdimming value; and when the set dimming degree based on the dimmingsignal is smaller than the lower limit dimming value, while the controlunit sets the output frequency of the high frequency power supply to afrequency higher than the dimming frequency, the control unit varies theoutput frequency from the test frequency to an upper limit frequency inresponse to the lower limit dimming value.

Also, the above-described control unit is characterized in that in thecase that a difference voltage is higher than, or equal to a presetthreshold value voltage, the difference voltage being produced between adetection voltage by the lamp voltage detecting circuit when a dimmingfrequency is outputted and another detection voltage by the lamp voltagedetecting circuit when a test frequency higher than the dimmingfrequency is outputted, the control unit sets a lower limit dimmingvalue; and in the case that the set dimming degree based upon thedimming signal is smaller than the lower limit dimming value, thecontrol unit sets the output frequency of the high frequency powersupply to an upper limit frequency in response to the lower limitdimming value.

Also, the above-described control unit is characterized in that in thecase that the difference voltage is higher than, or equal to the presetthreshold value voltage, the difference voltage being produced betweenthe detection voltage by the lamp voltage detecting circuit when thedimming frequency is outputted and another detection voltage by the lampvoltage detecting circuit when the test frequency higher than thedimming frequency is outputted, the control unit controls the outputfrequency of the high frequency power supply to become the dimmingfrequency.

Also, the above-described control unit is characterized in that in thecase that the difference voltage is higher than, or equal to the presetthreshold value voltage, the difference voltage being produced betweenthe detection voltage by the lamp voltage detecting circuit when thedimming frequency is outputted and another detection voltage by the lampvoltage detecting circuit when the test frequency higher than thedimming frequency is outputted, the control unit controls the outputfrequency of the high frequency power supply to be returned to thedimming frequency after controlling the output frequency for apredetermined time period in such a manner that the output frequencybecomes a current increase frequency lower than the dimming frequency.

Also, the above-described control unit is characterized in that thecontrol unit sets the lower limit dimming set value of the fluorescentlamp within a range from 5% to 60% with respect to a dimming degree whenthe rated power is entered to the fluorescent lamp in the normaltemperature.

Also, the above-described control unit is characterized in that thecontrol unit varies the lower limit dimming set value in response to thedetection voltage of the lamp voltage detecting circuit.

Also, the above-described control unit is characterized in that thecontrol unit sets a relationship between a time period T0 and a dimmingfrequency f1 to T0≧3/f1, in which a frequency is intermittently variedin the time period T0.

Also, the above-described control unit is characterized in that thecontrol unit sets time T0 during which a frequency is intermittentlyvaried to T0≧0.1 ms.

Furthermore, the above-described control unit is characterized in thatthe control unit sets both time T0 during which a frequency isintermittently varied and time T2 during which a frequency higher thanthe dimming frequency is outputted to T2≦T0/2.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram for indicating an arrangement of a dimmingapparatus for a fluorescent lamp according to an embodiment mode 1 ofthe present invention;

FIG. 2 is an operation flow chart of a control unit employed in theembodiment mode 1;

FIG. 3 is a waveform diagram of an output frequency of a high frequencypower supply 1 in the embodiment mode 1;

FIG. 4 is a waveform diagram of a lamp current flowing through afluorescent lamp 3 in the embodiment mode 1;

FIG. 5 is a waveform diagram of a lamp voltage applied to thefluorescent lamp 3 in a temperature of 25° C. in the embodiment mode 1;

FIG. 6 is a waveform diagram of a lamp voltage applied to thefluorescent lamp 3 in a temperature of 0° C. in the embodiment mode 1;

FIG. 7 is a graphic representation for indicating a relationship betweena lamp voltage and a lower limit setting value of dimming employed in anexpansion of the embodiment mode 1;

FIG. 8 is an operation flow chart of a control unit employed in anembodiment mode 2 of the present invention;

FIG. 9 is a waveform diagram of an output frequency of a high frequencypower supply 1 in the embodiment mode 2;

FIG. 10 is a waveform diagram of a lamp current flowing through afluorescent lamp 3 in the embodiment mode 2;

FIG. 11 is a waveform diagram of a lamp voltage applied to thefluorescent lamp 3 in a temperature of 25° C. in the embodiment mode 2;

FIG. 12 is a waveform diagram of a lamp voltage applied to thefluorescent lamp 3 in a temperature of 0° C. in the embodiment mode 2;

FIG. 13 is an operation flow chart of a control unit employed in anembodiment mode 3 of the present invention;

FIG. 14 is a waveform diagram of an output frequency of a high frequencypower supply 1 in a temperature of 25° C. in the embodiment mode 3;

FIG. 15 is a waveform diagram of a lamp voltage applied to thefluorescent lamp 3 in the temperature of 25° C. in the embodiment mode3;

FIG. 16 is a waveform diagram of an output frequency of a high frequencypower supply 1 in a temperature of 0° C. in the embodiment mode 3;

FIG. 17 is a waveform diagram of a lamp voltage applied to thefluorescent lamp 3 in the temperature of 0° C. in the embodiment mode 3;

FIG. 18 is an operation flow chart of a control unit employed in anembodiment mode 4 of the present invention;

FIG. 19 is a waveform diagram of an output frequency of a high frequencypower supply 1 in a temperature of 25° C. in the embodiment mode 4;

FIG. 20 is a waveform diagram of a lamp voltage applied to thefluorescent lamp 3 in the temperature of 25° C. in the embodiment mode4;

FIG. 21 is a waveform diagram of an output frequency of a high frequencypower supply 1 in a temperature of 0° C. in the embodiment mode 4;

FIG. 22 is a waveform diagram of a lamp voltage applied to thefluorescent lamp 3 in the temperature of 0° C. in the embodiment mode 4;

FIG. 23 is a block diagram for indicating the arrangement of theconventional dimming apparatus for the fluorescent lamp;

FIG. 24 is a characteristic diagram of the output frequency of the highfrequency power supply with respect to the set dimming degree; and

FIG. 25 is a characteristic diagram of a light output with respect tothe set dimming degree.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiment Mode 1.

FIG. 1 is a block diagram for representing a dimming apparatus of afluorescent lamp, according to an embodiment mode 1 of the presentinvention.

It should be noted that the same reference numerals shown in theconventional apparatus of FIG. 23 will be employed as these forindicating the same, or similar portions of the inventive apparatusindicated in FIG. 1. As new reference numeral 6 indicates a lamp voltagedetecting circuit for detecting a voltage of the lamp 3. A detectionvoltage of this lamp voltage detecting circuit 6 is entered into thecontrol unit 4. The control unit 4 intermittently controls the outputfrequency of the high frequency power supply 1 in such a manner that thefrequency-controlled output frequency of the high frequency power supply1 becomes higher than the dimming frequency obtained in response to thedimming signal 5, so that a current supplied to the fluorescent lamp 3is periodically changed into a low current. Also, in the case that theoutput frequency of the high frequency power supply 1 is higher than theabove-described dimming frequency, the control unit 4 sets an upperlimit frequency of the output frequency of the high frequency power 1based upon the detection voltage of the lamp voltage detecting circuit6, so that the output frequency of the high frequency power supply 1 iscontrolled to be lower than, or equal to this upper limit frequency.

Next, operations of the embodiment mode 1 will be explained withreference to a flow chart indicated in FIG. 2. FIG. 2 is an operationflow chart of the control unit 4, and when the power supply is turnedON, the operation of the control unit 4 is commenced from START.

At a step S1, the lower limit dimming setting operation is initialized.In the embodiment mode 1, a lower limit dimming set value DL is selectedto be 25% as an initial value. A dimming degree of 25% implies a lightoutput rate of the lamp 3. When the rated power is inputted to the lamp3 in the normal temperature (25° C.), the dimming rate is recognized as100%.

At a step S2, the dimming signal 5 which constitutes a set dimmingdegree is entered into the control unit 4 so as to acquire the setdimming degree as an internal signal DM.

At a step S3, the control unit 4 checks as to whether or not the setdimming degree DM is larger than, or smaller than a predetermineddimming degree. In the embodiment mode 1, this predetermined dimmingdegree is selected to be 50%. At the step S3, when the set dimmingdegree DM is higher than, or equal to 50% (no), the control operation isadvanced to a step S4, whereas when the set dimming degree DM is lowerthan, or equal to 50% (yes), the control operation is advanced toanother step S5.

At this step S4, the control unit 4 outputs a dimming frequency f1corresponding to the set dimming degree DM to the high frequency powersupply 1. When this process operation of the step S4 is accomplished,the control operation is returned to the step S2 at which the sameoperation is repeatedly carried out.

At the step S5, the control unit 4 checks as to whether or not the setdimming degree DM is larger than the lower limit dimming set value DL.When the set dimming degree DM is larger than the lower limit dimmingset value DL (no), the control operation is advanced to a step S6,whereas when the set dimming degree DM is smaller than the lower limitdimming set value DL (yes), the control operation is advanced to a stepS7.

At the step S6, the control unit 4 controls the high frequency powersupply 1 to output a frequency f2 higher than the dimming frequency f1.For the sake of an easy explanation, the frequency f2 is referred to asa test frequency. In this embodiment mode 1, the test frequency f2 isset to such a frequency equal to 20% of the set dimming degree.

At the step S7, the control unit 4 controls the high frequency powersupply 1 to output an upper limit frequency fd. The upper limitfrequency fd is equal to a frequency for setting a lower limit dimmingvalue, and is such a frequency equivalent to 25% of the set dimmingdegree when the power supply is turned ON. When the process operationdefined at the step S7, the control operation is returned to the stepS2.

At a step S8, a timer with predetermined time T2 is actuated, and whenthe timer set time T2 has passed, the control operation is advanced to astep S9.

At the step S9, the control unit 4 enters thereinto the detectionvoltage of the lamp voltage detecting circuit 6 to acquire thisdetection voltage as an internal signal VL.

At a step S10, the control unit 4 checks as to whether or not the lampvoltage VL is higher than a preset threshold value voltage VS. When thelamp voltage VL is lower than the threshold value voltage VS (no), thecontrol operation is advanced to a step S12, whereas when the lampvoltage VL is higher than the threshold value voltage VS (yes), thecontrol operation is advanced to a step S11.

At the step S11, the lower limit dimming setting operation is again set.In this embodiment mode 1, the lower limit dimming set value DL is againset to 40%.

At a step S12, the control unit 4 controls the high frequency powersupply 1 to output the dimming frequency f1.

At a step S13, the timer T1 is operated. When the timer set time T1 haspassed, the control operation is returned to the step S2.

Next, overall operations will now be explained with reference towaveform diagrams indicated in FIG. 3 to FIG. 6.

FIG. 3 is a schematic diagram for representing a transition of thefrequency of the high frequency power supply 1, FIG. 4 indicates awaveform of a lamp current flowing through the lamp 3, FIG. 5 shows awaveform of a lamp voltage at the normal temperature (25° C.), and FIG.6 represents a waveform of a lamp voltage in a low temperature (0° C.).Also, these drawings are waveform diagrams in such a case that thedimming signal 5 becomes lower than, or equal to the dimming degree of50%.

As shown in FIG. 3, in response to the control signal supplied from thecontrol unit 4, the frequency of the high frequency power supply 1repeatedly represents the dimming frequency f1 and the test frequency f2in an alternate manner. Since the impedance of the coil 2 is varied inaccordance with the frequency, when the frequency is changed from thedimming frequency f1 to the test frequency f2, the current flowingthrough the lamp 3 is decreased during the timer set time T2, asindicated in FIG. 4. Also, as shown in FIG. 5, the lamp voltage in thetemperature of 25° C. is increased in the time period T2 during whichthe lamp current is decreased, but is not reached to the threshold valueVS.

On the other hand, the lamp voltage in the temperature of 0° C. isincreased to a voltage exceeding the threshold value voltage VS for atime period of the timer setting time T2, as shown in FIG. 6. Aspreviously explained, in FIG. 2, as a result of judging of the lampvoltage at the step S10, when the lamp voltage VL becomes higher thanthe threshold value voltage VS, the lower limit dimming set value DL ischanged into 40% at the step S11.

As a result, thereafter, even when the dimming signal 5, namely the setdimming degree is smaller than, or equal to 40%, the control unit 4judges at a step S5 that the set dimming degree DM<the lower limitdimming set value DL. At a step S7, the high frequency power supply 1 isoperated in the frequency equivalent to the set dimming degree of 40%.As apparent from FIG. 25, the set dimming degree of 40% corresponds tosuch an area where the lamp 3 is turned ON under stable condition. Atthis time, the light output is equal to such a light output based on thelower limit dimming set value of 25% which is set by the initial settingoperation in the case of the normal temperature (25° C.) at the step S1.Even when the set dimming degree of 40% is set to the lower limit value,the light output from the lamp can be sufficiently dimmed.

As previously explained, the lamp 3 is not dimmed in the unstableturn-ON area even under such a low temperature as 0° C. As a result,there are no possibilities that the light output is rapidly lowered, andthe flicker phenomenon occurs. Also, since the lamp current isperiodically reduced in the repeating manner, if the repetitionfrequency becomes higher than, or equal to 50 Hz, this flickerphenomenon cannot be felt by human eyes.

It should be noted that in the embodiment mode 1, when the set dimmingdegree DM set by the dimming signal 5 is smaller than, or equal to 50%,the lamp current is decreased, namely, the control unit controls so thatthe test frequency f2 is output during a predetermined time period.Alternatively, even when the set dimming degree is not limited to 50%, asimilar effect may be achieved.

Also, in the embodiment mode 1, in such a case that the set dimmingdegree DM set by the dimming signal 5 is smaller than, or equal to 50%,and also while the set dimming degree DM is larger than the lower limitdimming set value DL, the lamp voltage VL is higher than the thresholdvalue VS, the lower limit dimming set value DL is changed into 40%.Alternatively, the lower limit dimming set value DL may be independentlyset, depending upon the use temperature range of the lamp 3 and the sortof this lamp 3. It is preferably set the lower limit dimming set valueDL within a range from 5% to 60% with respect to the dimming degreedefined when the rated power is inputted to the fluorescent lamp 3 inthe normal temperature.

Also, as indicated in FIG. 7 the lower limit dimming set value definedat the step S11 may be continuously varied from DL1 to DL2 in responseto the lamp voltage. In this alternative case, when the lamp voltage islow, the lower limit dimming set value is selected to be a low value. Inother words, when the lamp voltage is equal to VL1, the lower limitdimming set value is set to the lower limit dimming set value DL1. Onthe other hand, when the lamp voltage is high, the lower limit dimmingset value is selected to be a high value. In other words, when the lampvoltage is equal to VL2, the lower limit dimming set value is set to thelower limit dimming set value DL2. In this case, the processed contentof the step S11 shown in FIG. 2 must be changed into such a processedcontent. That is, the lower limit dimming set value is variable inresponse to the lamp voltage VL.

Also, in the waveform shown in FIG. 3, a relationship between the timeperiod T0 at which the frequency of the high frequency power supply 1 isintermittently changed, and the dimming frequency f1 is suitablyselected to be T0≧3/f1. The above-described time period T0 at which thefrequency is intermittently changed may be suitably selected to beT0≧0.1 ms. This time period T0 and another time period T2 may bepreferably selected to T2≦T0/2. In the time period T2, the testfrequency f2 higher than the dimming frequency f1 is outputted.

Embodiment Mode 2

Next, FIG. 8 is a flow chart for representing operations of a controlunit 4 according to an embodiment mode 2 corresponding to the flow chartof the embodiment mode 1 shown in FIG. 2. An arrangement according tothis embodiment mode 2 is equipped with the arrangement similar to thatof the embodiment mode 1 shown in FIG. 1.

Subsequently, operations related to the embodiment mode 2 will now beexplained with reference to the flow chart shown in FIG. 8.

First, since the power supply is turned ON, the operation of the controlunit 4 is commenced. Since operations defined from a step S21 to a stepS28 are carried out in a similar manner to the operations defined fromthe step S1 to the step S8 of the flow chart of the embodiment mode 1shown in FIG. 2, descriptions thereof are omitted.

At a step S29, a detection voltage of the lamp voltage detecting circuit6 is inputted into the control unit 4 so as to acquire this detectionvoltage as an internal signal V2.

At a step S30, the control unit 4 controls the high frequency powersupply 1 to output the dimming frequency f1.

At a step S31, the timer T1 is operated. When the timer set time haspassed, the control operation is returned to a step 32.

At the step S32, a lamp voltage is again applied from the lamp voltagedetecting circuit 6 to the control unit 4 so as to acquire this lampvoltage as an internal signal V1.

At a step S33, the control unit 4 calculates a difference voltage VDbetween a lamp voltage V1 and a lamp voltage V2 as an internal signal.

At a step S34, the control unit 4 checks as to whether or not thedifference voltage VD is lower than a preset threshold value voltage VS.When the difference voltage VD is higher than the threshold valuevoltage VS (yes), the control operation is advanced to a step S35.Conversely, when the difference voltage VD is lower than the thresholdvalue voltage VS, the control operation is returned to the step S22.

At a step S35, the lower limit dimming set value DL is set to apredetermined dimming degree, namely 40% in the embodiment mode 2similar to that of the embodiment mode 1.

Next, overall operations will now be explained with reference towaveform diagrams indicated in FIG. 9 to FIG. 12.

FIG. 9 is a schematic diagram for representing a transition of thefrequency of the high frequency power supply 1, FIG. 10 indicates awaveform of a lamp current flowing through the lamp 3, FIG. 11 shows awaveform of a lamp voltage at the normal temperature (25° C.), and FIG.12 represents a waveform of a lamp voltage in a low temperature (0° C.).Also, these drawings are waveform diagrams in such a case that thedimming signal 5 becomes lower than, or equal to the dimming degree of50%.

FIG. 9 and FIG. 10 are similar to those of the embodiment mode 1 shownin FIG. 3 and FIG. 4. In this embodiment mode 2, as represented as thevoltage waveforms of the high frequency power supply 1 shown in FIG. 11and FIG. 12, when the high frequency power supply 1 is operated at thedimming frequency f1, the lamp voltage becomes a lamp voltage V1,whereas when the high frequency power supply 1 is operated at the testfrequency f2, the lamp voltage becomes a lamp voltage V2. A VD becomes avoltage difference between both the lamp voltages V1 and V2.

Also, FIG. 11 shows the lamp voltage in such a case that the atmospherictemperature of the lamp 3 is equal to 25° C., and FIG. 12 indicates thelamp voltage in such a case that the atmospheric temperature of the lamp3 is equal to 0° C. As apparent from these drawings, when theatmospheric temperature is 0° C., the difference voltage VD is high. Ifthe threshold value voltage VS is set to be a proper value, namely, anintermediate value between the difference voltage VD in the normaltemperature (25° C.) and the difference voltage VD in the lowtemperature (0° C.), then the lower limit dimming set value in the lowertemperature can be set to 40% similar to the embodiment mode 1.

As previously described, at the step S34 of the flow chart shown in FIG.8, when the control unit 4 judges the difference voltage of the lampvoltages, in such a case that the difference voltage VD is higher thanthe threshold value voltage VS, the lower limit dimming set value DL isset to 40% at the step S35.

As a result, thereafter, even when the dimming signal 5, namely the setdimming degree is smaller than, or equal to 40%, the control unit 4judges at a step S25 that the set dimming degree DM<the lower limitdimming set value DL. At a step S27, the high frequency power supply 1is operated in the frequency equivalent to the set dimming degree of40%.

As previously explained, according to the embodiment mode 2, similar tothe embodiment mode 1, the lamp 3 is not dimmed in the unstable turn-ONarea even under such a low temperature as 0° C. As a result, there areno possibilities that the light output is rapidly lowered, and theflicker phenomenon occurs.

Embodiment Mode 3

Next, FIG. 13 is a flow chart for representing operations of a controlunit 4 according to an embodiment mode 3 corresponding to the flow chartof the embodiment mode 1 shown in FIG. 2. An arrangement according tothe embodiment mode 3 is equipped with the arrangement similar to thatof the embodiment mode 1 shown in FIG. 1.

Subsequently, operations related to the embodiment mode 3 will now beexplained with reference to the flow chart shown in FIG. 13.

First, since the power supply is turned ON, the operation of the controlunit 4 is commenced. Since operations defined from a step S41 to a stepS45 are carried out in a similar manner to the operations defined fromthe step S1 to the step S5 of the flow chart of the embodiment mode 1shown in FIG. 2, descriptions thereof are omitted.

At the step S45, the control unit 4 checks as to whether or not the setdimming degree DM is larger than the lower limit dimming set value DL.When the lower limit dimming set value DM is larger than the set dimmingdegree DL (no), the control operation is advanced to a step S46, whereaswhen the lower limit dimming set value DM is smaller than the setdimming degree DL (yes), the control operation is advanced to a stepS47.

At the step S46, the control unit 4 controls the high frequency powersupply 1 to output a dimming frequency f1.

At the step S47, the control unit 4 controls the high frequency powersupply 1 to output an upper limit frequency fd. Similar to theembodiment mode 1, the upper limit frequency fd is equal to a frequencyin correspondence with the lower limit dimming set value DL, and is sucha frequency equivalent to 25% of the set dimming degree during theinitial setting operation when the power supply is turned ON. When theprocess operation defined at the step S47, the control operation isreturned to the step S42.

At a step S48, a timer T1 is actuated, and when the timer set time T1has passed, the control operation is advanced to a step S49.

At the step S49, the control unit 4 enters thereinto the detectionvoltage of the lamp voltage detecting circuit 6 to acquire thisdetection voltage as an internal signal V1.

At a step S50, the control unit 4 controls the high frequency powersupply 1 to output the test frequency f2.

At a step S51, the timer T2 starts to be operated.

At a step S52, a detection voltage is again applied from the lampvoltage detecting circuit 6 to the control unit 4 so as to acquire thisdetection voltage as an internal signal V2.

At a step S53, the control unit 4 calculates a difference voltage VDbetween a lamp voltage V1 and a lamp voltage V2 as an internal signal.

At a step S54, the control unit 4 checks as to whether or not thedifference voltage VD is lower than a preset threshold value voltage VS.When the difference voltage VD is lower than, or equal to the thresholdvalue voltage VS (no), the control operation is advanced to a step S55.Conversely, when the difference voltage VD is higher than, or equal tothe threshold value voltage VS (yes), the control operation is advanceto the step S56.

At a step S55, the control unit 4 judges as to whether or not the timerT2 is accomplished (yes). When the timer T2 is accomplished, the controloperation is returned to the step S42. On the other hand, when the timerT2 is not yet accomplished, the control operations defined after thestep S52 are repeatedly carried out.

At a step S56, the lower limit dimming set value DL is changed to be40%.

At a step S57, the control unit 4 returns the present frequency to thedimming frequency f1 (otherwise upper limit frequency fd).

At a step S58, the control unit 4 judges as to whether or not the timerT2 is accomplished. When the timer T2 is accomplished (yes), the controloperation is returned to the step S42.

Next, overall operations will now be explained with reference towaveform diagrams indicated in FIG. 14 to FIG. 17.

FIG. 14 is a schematic diagram for representing a transition of thefrequency of the high frequency power supply 1 in the temperature of 25°C., FIG. 15 shows a waveform of a lamp voltage at the temperature of 25°C., FIG. 16 represents a schematic diagram of a transition of afrequency in the temperature of 0° C., and FIG. 17 indicates a lampvoltage in the temperature of 0° C.

As indicated in FIG. 14 and FIG. 15, since the difference voltage VD ofthe lamp voltage is low with respect to the threshold value voltage VSin the temperature of 25° C., the test frequency f2 is outputted fromthe high frequency power supply 1 during the timer T2.

On the other hand, as indicated in FIG. 16 and FIG. 17, in thetemperature of 0° C., since the difference voltage VD exceeds thethreshold value voltage VS while the timer T2 is actuated (VD≧VS), thepresent frequency is switched from the test frequency f2 to the dimmingfrequency f1. At this time, since the timer T2 is continuously actuatedwithout any interrupt, the same operation is repeatedly performed afterthe timer T2 is accomplished. As a result, the frequency change timeperiod T0 of the high frequency power supply 1 is always constant.

As previously explained, according to the embodiment mode 3, similar tothe embodiment mode 1, the lamp 3 is not dimmed in the unstable turn-ONarea even under such a low temperature as 0° C. As a result, there areno possibilities that the light output is rapidly lowered, and theflicker phenomenon occurs. Also, since the minimum time during which thelamp current is experimentally reduced is required, the lamp can beturned ON under more stable condition.

Embodiment Mode 4

Next, FIG. 18 is a flow chart for representing operations of a controlunit 4 according to an embodiment mode 4 corresponding to the flow chartof the embodiment mode 1 shown in FIG. 2. An arrangement according tothis embodiment mode 4 is equipped with the arrangement similar to thatof the embodiment mode 1 shown in FIG. 1.

Subsequently, operations related to the embodiment mode 4 will now beexplained with reference to the flow chart shown in FIG. 18.

First, in the embodiment mode 4 shown in FIG. 18, the completely sameoperations as defined from the step S41 to the step S56 as those of theembodiment mode 3 shown in FIG. 13 are carried out.

That is to say, at a step S54, the control unit 4 checks as to whetheror not a voltage difference VL is higher than a preset threshold valuevoltage VS. When the voltage difference VL is lower than the thresholdvalue voltage VS (no), the control operation is advanced to a step S55.Conversely, when the voltage difference VL is higher than the thresholdvalue voltage VS (yes), the control operation is advanced to a step S16.

At a step S55, the control unit 4 judges as to whether or not the timerT2 is accomplished. When the timer T2 is not yet accomplished, thecontrol operation is returned to the step S52. On the other hand, whenthe timer T2 is accomplished, the control operation is returned to thestep S42 at which the control operations defined from the first step S42are repeatedly carried out.

At a step S56, the lower limit dimming frequency DL is changed to be40%.

While the above-described control operations are similar to those of theembodiment mode 3, the below-mentioned control operations of theembodiment mode 4 are different therefrom.

At a step S60, the control unit 4 controls the high frequency powersupply 1 to output a current increase frequency f3. The current increasefrequency f3 is such a frequency lower than either the dimming frequencyor the upper limit frequency f1. This current increase frequency f3 maycause a substantially large current to forcibly flow through the lamp 3.This is because such an unstable operation occurred when the lampcurrent is decreased may be recovered to the stable condition by rapidlyincreasing the lamp current.

At a step S61 and a step S62, a new timer T3 is actuated. When the timerT3 is accomplished, the control unit 4 controls the high frequency powersupply 1 to output the dimming frequency f1 (otherwise, upper limitfrequency fd) at a step S63.

Furthermore, at a step S64, when the timer T2 is accomplished thecontrol operation is returned to the step S42 similar to the embodimentmode 3 shown in FIG. 13, at which a similar process operation isrepeatedly performed.

Next, overall operations will now be explained with reference towaveform diagrams indicated in FIG. 19 to FIG. 22.

FIG. 19 is a schematic diagram for representing a transition of afrequency in the temperature of 25° C. FIG. 20 shows a waveform of alamp voltage at the same temperature (25° C.). FIG. 21 is a schematicdiagram for showing a transition of a frequency in the temperature of 0°C., and FIG. 22 shows a lamp voltage at the same temperature (0° C.).

Since the difference voltage VD of the lamp voltage is low with respectto the threshold value voltage VS in the temperature of 25° C., a testfrequency f2 is outputted from the high frequency power supply 1 duringthe timer T2. On the other hand, in the temperature of 0° C., since thedifference voltage VL exceeds the threshold value voltage VS while thetimer T2 is actuated (VL≧VS), the present frequency is switched from thetest frequency f2 to the current increase frequency f3. Then, since thetimer 3 is actuated, the current increase frequency f3 is continuouslyoutputted.

When the timer 3 is accomplished, the dimming frequency f1 (otherwise,upper limit frequency fd) is outputted from the high frequency powersupply. At this time, since the timer T2 is continuously operatedwithout any interruption, after the timer T2 is accomplished, thecontrol operation is advanced to the step S42 at which the sameoperation is repeatedly carried out.

As a consequence, the frequency change time period T0 of the highfrequency power supply 1 is always constant.

As previously explained, according to the embodiment mode 4, similar tothe embodiment mode 1, the lamp 3 is not dimmed in the unstable lampturn-ON area even under such a low temperature as 0° C. As a result,there are no possibilities that the light output is rapidly lowered, andthe flicker phenomenon occurs. Also, since the minimum time during whichthe lamp current experimentally reduced is necessarily required, andfurthermore the lamp current is increased during the switchingoperation, the lamp can be turned ON under further stable condition.

As previously explained, in accordance with the present invention, thefrequency is increased in such a manner that the lamp current isintermittently decreased from the dimming lamp current, and the upperlimit control frequency is set from a change in the lamp current at thistime. As a consequence, the fluorescent lamp is dimmed up to such anarea that the fluorescent lamp can be turned ON under stable condition,and the limiter is actuated in the dimming degree lower than thisdimming operation. Therefore, there is no case that the lamp current israpidly decreased in the discontinuous manner. Also, it is possible toavoid that the flicker phenomenon occurs at this time.

In the case that the dimming degree set by the dimming signal is smallerthan, or equal to a predetermined dimming degree, the output frequencyof the high frequency power supply is intermittentlyfrequency-controlled in such a manner that the output frequency becomessuch a test frequency higher than the dimming frequency from the dimmingfrequency based upon the dimming signal so as to intermittently decreasethe lamp current. As a result, the intermittent change is carried outwithin the area where the lamp current is small, and thus, no sounds areproduced from the lamp and the like.

Also, since the upper limit control frequency is set, the upper limitfrequency value is variable, so that the light amount is not increasedwith the same dimming signal, as compared with that in the normaltemperature.

In the case that the lamp voltage difference between the lamp voltageduring the dimming operation and the lamp voltage when the lamp currentis decreased is detected, and then this detected lamp voltage differenceis a predetermined voltage difference, since the upper limit controlfrequency is set, there is no adverse influence caused by the individualdifferences in the lamp current/voltage characteristics.

Also, since the decrease of the lamp current is stopped when apredetermined lamp voltage difference is detected, and furthermore, thedecrease time of the lamp current is minimized, even when the lampcurrent is decreased, the stable discharge can be obtained.

Also, after the decrease of the lamp current has been stopped, thelarger current than the dimming current is supplied to the lamp during apredetermined time period and then is held for a predetermined timeperiod. As a result, even when the lamp current is decreased, thedischarge is furthermore performed under stable condition, so that theuse temperature range can be extended.

Also, since the upper limit frequency value is set to the dimmingdegrees from 5% to 60%, even when the sort of lamp is changed, thedimming apparatus can according to the present invention can turn ON thedifferent sorts of lamps under stable conditions.

Also, since the time period during which the lamp current isintermittently decreased is set to be longer than the time period of thedimming frequency more than, or equal to 3 times, even when the sort oflamp is changed, the dimming apparatus according to the presentinvention can turn ON the different sorts of lamps under stableconditions.

Also, since the time is set to be longer than, or equal to 0.1 ms,during which the lamp current is intermittently decreased, the upperlimit frequency value can be firmly set.

Furthermore, since the time period during which the lamp current isintermittently decreased is made more than, or equal to two times longerthan the time during which the frequency higher than the dimmingfrequency is outputted, the upper limit frequency value can be surelyset.

FIELD OF INDUSTRIAL APPLICATION

As previously described, in accordance with the present invention, thecontrol unit controls the output frequency of the high frequency powersupply in such a manner that this output frequency becomes higher thanthe dimming frequency based upon the entered dimming signal in order toperiodically lower the current supplied to the fluorescent lamp to thelow current. Also, when the output frequency of the high frequency powersupply becomes such a frequency higher than the dimming frequency, sincethe lower limit dimming set value is set based on the detection voltageof the lamp voltage detecting circuit, the above-described outputfrequency of the high frequency power supply is controlled to becomelower than, or equal to the above-explained upper limit frequency inresponse to the lower limit dimming set value. As a consequence, thelamp can be dimmed in the continuous manner, and the flicker phenomenonof the light output can be reduced. In particular, the lamp can beturned ON under stable condition in the low temperature.

What is claimed is:
 1. A dimming apparatus of a fluorescent lamp,comprising:a fluorescent lamp; a high frequency power supply forsupplying high frequency power to said fluorescent lamp; a control unitfor controlling an output frequency of said high frequency power supplyin response to an inputted light dimming signal; and a coil providedbetween said high frequency power supply and said fluorescent lamp, forlimiting a current which flows from said high frequency power supply tosaid fluorescent lamp; wherein: said dimming apparatus is furthercomprised of a lamp voltage detecting circuit for detecting a dischargevoltage of said fluorescent lamp; and said control unit intermittentlycontrols the output frequency of said high frequency power supply insuch a manner that said output frequency becomes higher than a dimmingfrequency based on said dimming signal so as to periodically change acurrent supplied to said fluorescent lamp into a low current; and whenthe output frequency of said high frequency power supply becomes higherthan said dimming frequency, said control unit sets a lower limitdimming set value based upon a detection voltage of said lamp voltagedetecting circuit in order that the output frequency of said highfrequency power supply is controlled to become lower than, or equal to aupper limit frequency in accordance with said lower limit dimming setvalue.
 2. A dimming apparatus of a fluorescent lamp as claimed in claim1 wherein:when a set dimming degree based upon said inputted dimmingsignal is smaller than, or equal to a predetermined dimming degree, saidcontrol unit intermittently controls the output frequency of said highfrequency power supply in such a manner that said output frequencybecomes a test frequency higher than said dimming frequency from thedimming frequency based upon said dimming signal.
 3. A dimming apparatusof a fluorescent lamp as claimed in claim 2 wherein:when the detectionvoltage of said lamp voltage detecting circuit is higher than a presetthreshold value voltage, said control unit sets a lower limit dimmingvalue; and when the set dimming degree based on said dimming signal issmaller than said lower limit dimming value, while the control unit setsthe output frequency of said high frequency power supply to a frequencyhigher than said dimming frequency, said control unit varies said outputfrequency from said test frequency to an upper limit frequency inresponse to said lower limit dimming value.
 4. A dimming apparatus of afluorescent lamp as claimed in claim 2 wherein:in the case that adifference voltage is higher than, or equal to a preset threshold valuevoltage, said difference voltage being produced between a detectionvoltage by said lamp voltage detecting circuit when a dimming frequencyis outputted and another detection voltage by said lamp voltagedetecting circuit when a test frequency higher than said dimmingfrequency is outputted, said control unit sets a lower limit dimmingvalue; and in the case that the set dimming degree based upon saiddimming signal is smaller than said lower limit dimming value, saidcontrol unit sets the output frequency of said high frequency powersupply to an upper limit frequency in response to said lower limitdimming value.
 5. A dimming apparatus of a fluorescent lamp as claimedin claim 4 wherein:in the case that said difference voltage is higherthan, or equal to said preset threshold value voltage, said differencevoltage being produced between said detection voltage by said lampvoltage detecting circuit when said dimming frequency is outputted andanother detection voltage by said lamp voltage detecting circuit whensaid test frequency higher than said dimming frequency is outputted,said control unit controls the output frequency of the high frequencypower supply to become said dimming frequency.
 6. A dimming apparatus ofa fluorescent lamp as claimed in claim 4 wherein:in the case that saiddifference voltage is higher than, or equal to said preset thresholdvalue voltage, said difference voltage being produced between saiddetection voltage by said lamp voltage detecting circuit when saiddimming frequency is outputted and another detection voltage by saidlamp voltage detecting circuit when said test frequency higher than saiddimming frequency is outputted, said control unit controls the outputfrequency of said high frequency power supply to be returned to saiddimming frequency after controlling the output frequency for apredetermined time period in such a manner that said output frequencybecomes a current increase frequency lower than said dimming frequency.7. A dimming apparatus of a fluorescent lamp as claimed in claim 1wherein:said control unit sets said lower limit dimming set value ofsaid fluorescent lamp within a range from 5% to 60% with respect to adimming degree when the rated power is entered to said fluorescent lampin the normal temperature.
 8. A dimming apparatus of a fluorescent lampas claimed in claim 1 wherein:said control unit varies the lower limitdimming set value in response to the detection voltage of said lampvoltage detecting circuit.
 9. A dimming apparatus of a fluorescent lampas claimed in claim 1 wherein:said control unit sets a relationshipbetween a time period T0 and a dimming frequency f1 to T0≧3/f1, in whicha frequency is intermittently varied in said time period T0.
 10. Adimming apparatus of a fluorescent lamp as claimed in claim 1wherein:said control unit sets time T0 during which a frequency isintermittently varied to T0≧0.1 ms.
 11. A dimming apparatus of afluorescent lamp as claimed in claim 1 wherein:said control unit setsboth time T0 during which a frequency is intermittently varied and timeT2 during which a frequency higher than said dimming frequency isoutputted to T2≦T0/2.